Process for Conversion of Natural Gas to Syngas Using a Solid Oxidizing Agent

ABSTRACT

A process for the conversion of natural gas to syngas. The process uses a solid oxidizing agent in place of an oxidizing gas for the partial oxidation of the natural gas.

BACKGROUND OF THE INVENTION

The present invention relates to a process for converting natural gasinto other commercial products. Specifically, the invention relates tothe production of syngas from natural gas using a solid oxidizing agent.

Natural gas generally refers to light gaseous hydrocarbons, andespecially comprising methane. Natural gas also contains hydrocarbonssuch as ethane, propane, butanes, and the like. Natural gas is recoveredfrom underground reservoirs, and is commonly used as an energy sourcefor heating and power generation. Typically, natural gas is recovered athigh pressure, processed and fed into a gas pipeline under pressure.Natural gas can comprise undesirable components, such as carbon dioxide,nitrogen and water, which can be removed with technology commonlyavailable. One example is the use of adsorbents for removingnon-hydrocarbon components of the natural gas, and or sulfur compounds.

Natural gas is usually processed to recover heavier hydrocarboncomponents found in the natural gas, and to increase the relativemethane content. Components recovered from natural gas include ethane,propane, butanes, and the like, as well as unsaturated hydrocarbons,leaving methane as the principal component of the processed natural gas.

Natural gas is most commonly handled in gaseous form, and transported bypipeline to processing plants, and then onto gas pipelines fortransmission and distribution. However, there is much natural gas thatis located in remote locations, and needs to be transported without theability to feed the natural gas into a pipeline. In addition naturalgas, or more precisely methane, can be processed to produce highermolecular weight hydrocarbon products for use as liquid fuels,lubricants, or monomers for plastics.

The need for methods of processing methane can improve the recovery anddistribution of natural gas, especially when the natural gas is situatedin distant and remote locations where the economics depend on how thenatural gas is brought to market.

SUMMARY OF THE INVENTION

The invention is a new process for generating a syngas from natural gas.The process comprises contacting a natural gas stream with an oxidizedsolid material in a reaction zone, and under reaction conditions togenerate a syngas. A reduced solid material is a result of the reaction,and the reduced solid material is regenerated in a regeneration zone.The reduced solid material is regenerated through oxidizing the solidmaterial under reaction conditions thereby generating the oxidized solidmaterial.

In one embodiment, the process comprises adding steam to the reactionzone to further increase the hydrogen content of the syngas. Theformation of syngas is a high temperature reaction with the reactionconditions including a temperature between 400° C. and 900° C. and apressure between 0.103 MPa (15 psia) and 6.9 MPa (1000 psia).

Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art from the following detaileddescription and drawing.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a diagram of a reactor for the process of convertingmethane to syngas using solid oxidants.

DETAILED DESCRIPTION OF THE INVENTION

Natural gas is traditionally collected and transported to plants forprocessing. The primary use of natural gas is for heating, and isprocessed by removing water, inert gases, and natural gas liquids, orhigher molecular weight hydrocarbons found in natural gas. The naturalgas is then compressed, or liquefied for transport. However, one newtechnology is to convert natural gas to methanol for transport as aliquid. This saves on compression costs, and/or liquefaction costs, andprovides for a safer material to transport.

Another process for changing the traditional compression andliquefaction of natural gas, is to convert the natural gas to syngas, orsynthesis gas. The first steps will be to remove inert components in thenatural gas, such as nitrogen, argon, and carbon dioxide. Natural gasliquids will also be recovered and directed to other processing ortransport. The treated natural gas will comprise primarily methane andsome ethane with small amounts of higher alkanes, such as propane.Preferably, the natural gas comprises more than 90% methane. Syngas canprovide for the generation of liquids from the methane. There are twoprimary methods of producing syngas from methane. One method is steamreforming where methane and steam react to form carbon monoxide andhydrogen. Steam reforming is energy intensive in that the processconsumes over 200 kJ/mole of methane consumed and therefore requires afurnace or other source of continuous heat. A second method is partialoxidation. Partial oxidation comprises burning methane in an oxygen leanenvironment where the methane is partially oxidized to carbon monoxidealong with the production of hydrogen and some steam. Partial oxidationis exothermic and yields a significant amount of heat. Because onereaction is endothermic and the other is exothermic, these reactions areoften performed together for efficient energy usage. Combining the steamreforming and partial oxidation yields a third process wherein the heatgenerated by the partial oxidation is used to drive the steam reformingto yield a syngas. However, the partial oxidation needs a higherconcentration of oxygen than is found in air and the energy associatedwith the separation of air off-sets the advantage of the energy neededfor steam reforming.

Processed for syngas formation are well known and can be found in U.S.Pat. Nos. 7,262,334 and 7,226,548, and are incorporated by reference intheir entirety. The resulting syngas comprises carbon monoxide (CO),water (H₂O), and hydrogen (H₂). The syngas can be catalyticallyconverted to larger hydrocarbons through Fischer-Tropsch synthesis.Fisher-Tropsch synthesis is a known process for the conversion ofoxidized carbon to hydrocarbon liquids, as shown in U.S. Pat. No.4,945,116. Typically the oxidized carbon is carbon monoxide and thesource is from the partial combustion of coal.

The oxidation of hydrocarbons can be carried out with a catalyst such asfor the production of butane to maleic anhydride or propylene toacrolein, as shown in U.S. Pat. Nos. 6,437,193 and 6,310,240. Theseprocesses are for the insertion of oxygen into a hydrocarbon to producea desirable oxygenate. The aim of partial combustion of a lighthydrocarbon, such as methane, is to strip all of the hydrogen from thehydrocarbon and to produce a gas of CO and H₂ for subsequent generationof larger molecules. While the transport mechanism shows that some ofthe oxygen can come from solids bearing the oxygen, the processes areoperated at lower temperatures than partial oxidation for the productionof syngas. Indeed, the processes show that at high temperatures thesolids are readily reoxidized for regeneration at temperature around500° C., indicating that the equilibrium of metals with their oxides isunfavorable at higher temperatures.

However, by controlling the process and by not adding any gaseous oxygenas in the references, and by having the oxygen from the solid oxidestaken away with the carbon atoms during the partial combustion, it wasfound that a favorable control over the production of syngas is achievedthrough the use of a solid oxidizing agent in a co-current reactor.

The present invention uses a solid oxidizing agent for the partialoxidation of methane. The advantage with this method is that during theprocess if there is over oxidation of the methane to produce carbondioxide (CO₂), the process is simultaneously reducing the solidoxidizing agent, and as the product comprising carbon dioxide andreduced solid oxiding agent progress through the reactor, the water gasshift equilibrium will reduce the carbon dioxide to carbon monoxide(CO).

The process comprises contacting a natural gas stream with an oxidizedsolid material in a reaction zone, thereby generating a syngas and areduced solid material. The reduced solid material and syngas areseparated, and the reduced solid material is passed to a regenerationzone. In the regeneration zone, the reduced solid material isregenerated through a reaction with an oxidizing gas thereby generatingthe oxidized solid material.

The process can be shown with respect to a looping reactor for use ingenerating the syngas. The reactor 10, as shown in the FIGURE, is acocurrent flow reactor, and comprises a reaction section 20, and aregeneration section 30. The oxidized solid material is heated and fedto the reaction section 20 through a solid feed conduit 22. Heat isadded to the process through the heated solid material. Methane, ornatural gas, is fed to the reaction section 20 through a natural gasconduit 24. The methane and the oxidized solid material travelcocurrently up the reaction section 20 where the syngas is formed. Theoxidized solid material is reduced to a reduced solid material and thesyngas and reduced solid material separate in a separation section 26.The syngas is directed through a product conduit 28 and the reducedsolid material is falls down the reactor separation section 26. Thereduced solid material is directed through a conduit 32 to theregeneration section 30. In an alternate embodiment, the process caninclude adding steam to the reaction section 20. The steam can be addedwith the oxidized solid material through the solid feed conduit 22,thereby facilitating the transport of the oxidized solid material, orthe steam can be added with the natural gas through the natural gasconduit 24, or the steam can be added through an independent port (notshown) for more individual control over the amount of steam added to theprocess. Steam also provides heat that can facilitate the reactions toproduce syngas.

The formation of syngas is a high temperature reaction with thetemperature between 500° C. and 900° C., and preferably between 600° C.and 850° C. The reaction conditions include a pressure in the reactor isbetween 0.103 MPa (15 psia) and 6.9 MPa (1000 psia), and preferablybetween 1.72 MPa (250 psia) and 4.14 MPa (600 psia).

In the regeneration section 30, an oxidizing gas is admitted to thesection 30 through an oxidizing gas inlet 34. The oxidizing gas cancomprise air or oxygen. The oxidizing agent needs to contain oxygen, asthe oxygen will be transferred to the syngas during the reaction withnatural gas. The oxidizing gas can further include steam. The steamprovides several advantages to the regeneration process. The steamprovides heat, and increases the volume of gas that facilitates liftingthe solid through the regeneration section 30.

The oxidized solid material is in a granular form and is a metal oxide.The metal oxide comprises a metal selected from the group of alkalimetals, alkaline earth metals, transition metals, and mixtures of metalsfrom these groups. A preferred group of metals used are iron (Fe),nickel (Ni), copper (Cu), zinc (Zn), manganese (Mn), cerium (Ce),calcium (Ca), vanadium (V), niobium (Nb), tantalum (Ta), titanium (Ti),zirconium (Zr), hafnium (Hf), yttrium (Y), thorium (Th), lanthanum (La),and neodymium (Nd).

In another embodiment, the process comprises contacting the natural gasstream with a solid oxide material and a hydrocarbon activation materialunder reaction conditions, thereby generating a syngas stream and areduced solid material. The solid oxide, natural gas and hydrocarbonactivation material are fed into a reactor and carried co-currentlythrough the reactor. After exiting the reactor the reduced solid andhydrocarbon activation material are separated from the syngas anddirected to a regeneration zone for reoxidizing the reduced solid,thereby regenerating the solid oxide for reuse in the reactor.

The reaction conditions for temperature and pressure for this embodimentare as above.

The hydrocarbon activation material is a material found to activate themethane and to facilitate the reaction for syngas formation. Theactivation material is a solid, metal that is present in the reactor andis carried with the solid oxidizing agent. The hydrocarbon activationmaterial is a metal selected from one or more of: chromium (Cr),molybdenum (Mo), tungsten (W), manganese (Mn), technetium (Te), rhenium(Re), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), platinum(Pt), palladium (Pd), rhodium (Rh), iridium (Ir), and osmium (Os).Preferably, the activation material is selected from one or more ofchromium (Cr), molybdenum (Mo), tungsten (W), nickel (Ni), ruthenium(Ru), platinum (Pt), palladium (Pd), rhodium (Rh), and iridium (Ir).

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. A process for producing a syngas from natural gas comprising:contacting a natural gas stream with an oxidized solid material in areaction zone under reaction conditions, where the reaction temperatureis greater than 500° C. thereby generating a syngas and a reduced solidmaterial; passing the reduced solid material to a regeneration zone; andcontacting the reduced solid material with an oxidizing gas underreaction conditions thereby regenerating the oxidized solid material. 2.The process of claim 1 wherein the oxidizing gas comprises air oroxygen.
 3. The process of claim 2 wherein the oxidizing gas furthercomprises steam.
 4. The process of claim 1 further comprising feedingsteam into the reaction zone.
 5. The process of claim 1 wherein theoxidizes solid material is a granular form of a metal oxide, wherein themetal in the metal oxide is selected from the group consisting of alkalimetals, alkaline earth metals, transition metals, and mixtures thereof.6. The process of claim 5 wherein the metal in the metal oxide isselected from the group consisting of manganese (Mn), iron (Fe), nickel(Ni), copper (Cu), zinc (Zn), titanium (Ti), vanadium (V), and mixturesthereof.
 7. The process of claim 1 wherein the reaction conditions inthe reaction zone include a temperature between 600° C. and 850° C. 8.The process of claim 1 wherein the reaction conditions in the reactionzone include a pressure between 0.103 MPa (15 psia) and 6.9 MPa (1000psia).
 9. The process of claim 1 wherein the natural gas comprises morethan 90% methane.
 10. The process of claim 1 wherein the syngascomprises H₂ and CO.
 11. The process of claim 1 wherein the oxidizedsolid material and the natural gas are carried cocurrently in thereactor.
 12. A process for producing a syngas from natural gascomprising: contacting a natural gas stream to an oxidation process inthe presence of a solid oxide material and a hydrocarbon activationmaterial under reaction conditions, where the reaction temperature isgreater than 500° C. thereby generating a syngas and a reduced solidoxide material, where the natural gas and the solid oxide material arecarried cocurrently in a reactor; passing the reduced solid oxidematerial to a regeneration zone; and oxidizing the reduced solid oxidematerial with an oxidizing material thereby regenerating the solid oxidematerial.
 13. The process of claim 12 wherein the oxidizing gascomprises air or oxygen.
 14. The process of claim 13 wherein theoxidizing gas further comprises steam.
 15. The process of claim 12wherein the oxidizes solid material is a granular form of a metal oxide,and where the metal in the metal oxide is selected from the groupconsisting of alkali metals, alkaline earth metals, transition metals,and mixtures thereof.
 16. The process of claim 15 wherein the metal inthe metal oxide is selected from the group consisting of manganese (Mn),iron (Fe), copper (Cu), nickel (Ni), zinc (Zn), titanium (Ti), vanadium(V), and mixtures thereof.
 17. The process of claim 12 wherein thehydrocarbon activation material is selected from the group consisting ofchromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),molybdenum (Mo), technetium (Te), ruthenium (Ru), rhodium (Rh),palladium (Pd), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir),platinum (Pt), and mixtures thereof.
 18. The process of claim 12 whereinthe reaction conditions in the reaction zone include a temperaturebetween 600° C. and 850° C.
 19. The process of claim 12 wherein thereaction conditions in the reaction zone include a pressure between0.103 MPa (15 psia) and 6.9 MPa (1000 psia).